Why is the size of my backup bigger than the size of my filestore?

Since Artifactory uses checksum based storage, each file that exists in Artifactory is stored on the disk only once (even if the file exists in multiple locations).

However, on the backup, they are not saved once they are saved in each location they are referenced.

You can estimate the file size by looking at the Storage Summary (Admin -> Advanced -> Storage Summary) by comparing the Binaries Size with the Artifacts Size.